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| Author | Topic: Surging |
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yori Senior Member Posts: 8 |
posted October 06, 2005 03:54 AM
   
an extruder that has been runnung with pressure fluctations of about 5bars for a long time has started exhibiting surging after changing one of the materials in the blend (HDPE). the materials are very simmilar, and I am looking for a change in screw design that will make it less susceptible to minor variations in the material (grooved feed, barrier screw). Any suggestions? ------------------ IP: Logged |
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Tom C Moderator Posts: 625 |
posted October 06, 2005 10:05 PM
Differences in levels of crystalization and shear viscosity can cause the same MI resins to run differently in an extruder. Properly designed and operated barrier screws can increase the stable operating range of an extruder. Grooves tend to decrease stability over a wide operating range. Many have gone to gear pumps to achieve pressure stability. Some recent work using starve feeding and a specialized screw design have shown dramatic improvements in pressure stability. This is snew and patented technology. Please email me if you are interested in more details on this technique. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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Bob Cunningham Senior Member Posts: 115 |
posted October 09, 2005 08:02 AM
Is your pressure probe close to the screw? It is possible that your probe is seeing surging due to screw rotation, which goes away by the time the polymer reaches the die. -Bob Cunningham IP: Logged |
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yori Senior Member Posts: 8 |
posted October 09, 2005 08:30 AM
Thanks Bob, The surging cycle is about half a minute - it does not coincide with the screw rotation (different time scale). Yori IP: Logged |
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Steve H Moderator Posts: 374 |
posted October 09, 2005 03:50 PM
Have you experimented with some slight changes in the first and second barrel temps, and checked the drive controller on your extruder is maintaining constant screw speed. IP: Logged |
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yori Senior Member Posts: 8 |
posted October 10, 2005 01:43 AM
Thanks Steve, Changes in the profile, mainly heating the feed section, have helped. However, I am looking for screw design factors that effect surging. This screw is experimental. It has better temperature control and enables higher output. The aim is to design a new screw, that will have all the benefits and no surging. How can this be acheived? IP: Logged |
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Tom C Moderator Posts: 625 |
posted October 10, 2005 09:05 AM
Yori, If your screw were fed with melt at a constant temperature, and the barrel temperatures were held constant, and the RPMs were held constant it is likely that you would expereince little pressure flucuation. The trouble comes when you start with a solid material. The primary cause of surging is an acceleration of the melt becasue the solids are blocking the channel. Eventually the force of the melt rips apart the solid bed, opens more room for the melt, and the melt slows down. After a short period of time the original acceleration of the melt is restablished again. The acceleration and decceleration causes the surge. A similar phenomenon can occur in the metering section called "Solid bed breakup". An extrusion simulation program can be used to design screws and check the performance over a range of materials. Melt velocity and solids bed velocity are monitored to control surging. The one program I know that calculates these values is SPR's Extrud, which I use for design work.
Tom Cunningham
Extrusion Technical Services [This message has been edited by Tom C (edited October 10, 2005).] IP: Logged |
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TomBlack Moderator Posts: 56 |
posted October 11, 2005 07:16 AM
Yori, Melt pressure is developed in the screw metering section w/ a cubic relationship to depth and linear relationship to length. Provided you have the ability to deal w/ what may potentially be a slightly higher melt temperature (or faster screw speed requirement for the same output), then a shallower and longer pumping section should help dampen your pressure pertubatios... ------------------ www.ppsincorp.com IP: Logged |
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griffex Senior Member Posts: 31 |
posted October 23, 2005 03:41 PM
Dear Yori Please stop looking for a new screw to "solve" the problem. If you buy any new equipment at all, get a gear pump properly sized and matched to expected pressures. In the meantime, use rear-barrel temperature (first zone after separate grooved section), and quantify surge in terms of thickness variation as well as bars. You said it used to be 5 bar, but what is it when it surges? If pressure is measured at screw tip, where it should be (for safety reasons if nothing else) then you will see variations which usually don't mean anything, as one user already noted. Finally, what are you blending with the HDPE, and what was the nature of the change that started all this trouble? I hope this helps. Allan Griff www.griffex.com IP: Logged |
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mademan5284 Senior Member Posts: 20 |
posted October 24, 2005 10:33 AM
To me it sounds like you are running too cold for your speed. What I do to get rid of surging is look at my first two barrel zones Then I Just raise or lower the heats so that there is less difference between them. After that slow your speed down one tenth to one rpm at a time depending on how fast you are running already. However the srcew design that Tom mentioned makes sense if you want to solve your problem that way. You should get with him and run some diagnostic programs. IP: Logged |
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